This paper deals with material requirement planning (MRP) for a two levels production and assembly system with several types of items and one type of product, in multi periods. In this paper, we assume that items lead-times are probabilistic. An MRP approach with periodic order quantity (POQ) policy is used for the planning of items. The objective is to minimise the sum of total holding cost for all items, final product holding cost, final product backlogging and setup costs. The main policies in this model determine the periodic order quantity, order quantity and planned lead-times. The method to the mathematical model developed can be used for the optimisation of planned lead-times and periodicity for MRP systems under uncertain lead-time.